Glass-ceramic top cooking appliances incorporating radiant electric heaters have been known for many years. Throughout their period of development there has been an ongoing demand to reduce the heat-up time to radiance of heating elements in heaters used therein, when such heaters are energised.
A particularly fast heat-up time has been achieved with the use of tungsten-halogen lamps as heating elements, but such lamps are expensive.
Particular attention has therefore been given to the ongoing development of heaters incorporating bare metallic heating elements in the form of coiled wire or, more recently, corrugated metal ribbon. With such elements, oxidation of the components thereof occurs during operation thereof and the rate of such oxidation increases with increasing operating temperature. After prolonged periods of operation, failure of the elements occurs.
The use of a material such as iron-chromium-aluminium alloy for heater elements has led to the provision of heaters with good operating life expectancy of at least 2500 hours, as a result of the formation of a protective layer of aluminium oxide on the surface of the elements. However, even with the use of such a material care has to be taken to limit the operating temperature of the elements in order to achieve satisfactory operating life. It has been found, for example, that in the case of a corrugated ribbon heating element an increase in operating temperature by about 30.degree. C. at temperatures of about 1000.degree. C. can result in the life of the element being approximately halved.
For this reason, heaters are generally designed such that the element or elements therein operate in service at a temperature which will provide a predetermined operating life expectancy for the heater before failure of the element or elements occurs. Such operating temperature in service will be referred to in this specification as the predetermined optimum operating temperature of the element or elements which may, for example, be between about 960.degree. C. and about 1020.degree. C. for a typical corrugated ribbon element and between about 950.degree. C. and about 1150.degree. C. for a typical coiled wire element, according to the specific material, geometry and power of the ribbon or wire element, and may be selected, for example, to provide a predetermined operating life expectancy of 2500 hours or more.
It is particularly desirable to provide a heater in which the heat-up time to radiance is as short as possible and the temperature of the element or elements is as high as possible at least during an initial period, for example in order to promote rapid boiling of the contents of a cooking utensil placed on the glass-ceramic cook-top. Fast heat-up to radiance is also visually appealing to the user.